How come Walder Frey agreed to have Robb marry one of his daughters at the end of the war?

[Universal Sword Donor]

2 minutes ago, Tywin's Wallet said:

I disagree. Simply put - Frey was never going to give Robb a daughter - a red wedding was always gonna be the plan. The bethrotal was a tactic to give him time to plot betrayal with the Lannisters. Siding with the traitors is way too much risk for an upstart house.  He simply gave Robb a false sense of security by pretending to be on his side so he couldn't doubt him.  He was always going to off him.  Frey puts lots of value on family becuase to him it's a means of advancement, he was not about go waste one of his daughters  and all the alligiances he's had forged with the Lannisters on a rebel king for a cause he doesn't care about. 

There's nothing to substantiate that. If he never intended to let the wedding happen, why did he:

• Let Stevron go west with Robb and the cavalry, where he died
• Send 3k Frey infantry with Roose south, where many were killed and 4 Frey sons captured.
• Ransom the Freys at Harrenhal with gold

If Walder didn't intend to let Robb marry a Frey and was intent on betraying him from the get go, he sacrificed a lot of family to do so, quite unnecessarily.

[TheNecromancerofMirkwood]

Simple, really. Walder Frey is a weaselly scumbag who wanted to keep his options open. As others have said, if Robb started losing the war, then the Freys could jump ship and come out (relatively) clean. Its called Playing Both Sides.